For example, Patent Literature 1 discloses a fuel degradation detector including: detecting means for detecting a degree of oxidative degradation of biomass fuel derived from vegetable materials such as sugar cane, corn, palm oil, rapeseed, coconut, and soybean; and warning means for warning a driver when the detected degree of oxidative degradation exceeds a threshold. A fatty acid ester biomass fuel may release an acid by reaction with oxygen in air. The released acid causes metallic corrosion or the like of engine parts of a vehicle. Therefore, the device disclosed in Patent Literature 1 measures an acid value in a fuel tank and turns on a warning light when the measured acid value exceeds a threshold, thereby enabling a driver to be informed that some countermeasures should be taken before the metallic corrosion or the like of the engine parts is caused.